[feature] Custom function/map to translate selected text to link target/url

[lervag]

This was suggested by @BonaBeavis in #12.

The idea is to allow a custom map or function that is applied in order to transform the selected text when creating a new link.

Presently, if one selects Some text and creates a link with the <cr> visual mode mapping, the link becomes [[Some text]], i.e. the same text is used for the link target. The proposal is to allow a user to customize this, e.g. with:

let g:wiki_link_target_map = "MyFunction"

function MyFunction(text) abort
  return substitute(tolower(a:text), '\s\+', '-', 'g')
endfunction

With this configuration, the result of pressing <cr> on Some text would now be [[some-text|Some text]].

[lervag]

Relevant parts of the code include wiki#link#toggle, wiki#link#toggle_visual, and wiki#link#toggle_operator, as well as s:parser_word() in the same file.

[lervag]

@BonaBeavis Not sure if you still care about this plugin, but if you do then I have finally added the text to url map feature. I would be happy to hear if you think it works as expected.